1. Field of the Invention
The present invention relates to a video decoding method and an apparatus.
2. Description of the Related Art
A video encoding international standard system such as MPEG-2 (ISO/IEC 13818-2| ITU-T Rec. H.262), MPEG-4 (ISO/IEC 14496-2), or H.264 (ITU-T Rec. H.2641 ISO/IEC 14496-10) adopts an inter-predictive encoding such as inter-frame or inter-field predictive encoding. In a decoding process of encoded video data based on inter-predictive encoding, a predictive picture is generated by motion compensation using a plurality of reference pictures of a plurality of frames stored in a picture memory.
A picture memory for storing the reference pictures uses an external memory such as DRAM or an internal memory built-in in a decoding LSI. When a predictive picture is generated, the picture memory is frequently accessed. Thus, a broad memory bandwidth is needed. In order for the memory bandwidth to be increased, the number of the pins of LSI must be increased due to increase of a data bit width, or the number of external memories or an operating frequency must be increased. As a result, implementation costs increase.
There is proposed a technique of compressing a decoded picture and storing the compressed decoded picture in a picture memory in order to reduce frequency of the memory access, that is, the memory bandwidth. In this technique, when the decoded picture stored in the picture memory is used as a reference picture or a reconstructed picture for display, it is read out from the picture memory and decompressed. According to, for example, JP-A 2000-50272 (KOKAI) and JP-A 2000-78568 (KOKAI), the decoded picture is converted into a compressed picture by filtering and then stored to the picture memory. The reference picture is generated by decompressing the compressed picture read from the picture memory.
According to the JP-A 2000-04440 (KOKAI), the decoding process is done based on the compressed picture using only low frequency components of video encoded data. The reference picture is generated by compressing the decoded picture by Hadamard transform and quantization, and is stored in the picture memory. At the time of decoding, the compressed reference picture read from the picture memory is decompressed by dequantization and inverse Hadamard transform.
In the memory bandwidth reduction technique described by the above conventional arts, filtering or a process of irreversible compression and decompression such as Hadamard transform and quantization is subjected to the decoded picture for generating the reference picture. As a result, the compression distortion due to the decoding process is superposed on the reference picture. In the case of a video encoding system such as MPEG, the distortion of the reference picture is superposed on the next decoded picture, and the decoded picture on which the distortion is superimposed is used as a next reference picture. In this way, the distortion occurring at the time of decoding is accumulated over time, and is felt as a big noise in a reconstructed picture.